The invention has as its object an electrochromic system, in particular a transmission glazing of the electrochromic type or more precisely a laminated glazing whose light transmission is modified by application of an electric potential to the terminals of the glazing. The glazings according to the invention are used to control the solar lighting in a building or in the passenger space of a motor vehicle, in particular a vehicle equipped with a glass roof.
From patent applications EP-A-253713 and EP-89400814, a laminated electrochromic glazing is known which consists of two glass sheets coated with transparent electrically conductive layers--for example, indium oxide layers doped with tin (ITO)--separated successively by a layer of a cathode electrochromic material such as tungsten oxide (WO.sub.3), an electrolyte with proton conduction--for example, an electrolyte of orthophosphoric acid (H.sub.3 PO.sub.4) which has been uniformly dispersed in a poly(ethylene oxide) (PEO) film, and a layer of an anode electrochromic material such as iridium oxide. The two layers in contact with the electrolyte are capable of reversibly inserting protons if a suitable potential difference is applied to the two opposite ends of the glazing, the insertion reaction in the tungsten oxide layer corresponding to a deinsertion reaction in the iridium oxide layer which thus plays the role of a symmetrical counterelectrode of the tungsten. The thermodynamic balances can be written in the following manner: EQU WO.sub.3 +xH.sup.+ +xe.sup.-.rarw..fwdarw.HxWO.sub.3
and EQU HxIrOy .rarw..fwdarw.IrOy+xH.sup.+ +xe.sup.-
The instantaneous measurement of intensity I of the current which goes through the glazing is therefore a direct measurement of the number of insertion/deinsertion reaction sites which occur at this same time.
The applied potential difference should be greater, in absolute value, than the difference of the thermodynamic potentials of the insertion or deinsertion reaction of the protons. The greater the applied potential difference, the faster the coloring--or the fading--will be. However, beyond a certain voltage, there is the danger of noise occurring, particularly the reduction of molecular hydrogen of the proton or the oxidizing into oxygen of the water present in the residual state in some layers.
Taking into account the excess voltages in the interfaces, the limits of electrochemical stability characteristic of the system described above are between 0.6 and 1.5 volts in coloring phase (insertion of protons in WO.sub.3, deinsertion in HxIrOy) and between -0.6 and 0 volt in fading phase. In this description, these limits are designated below as "electrochemical stability range of the coloring and fading reaction."
The second problem which is posed is the time necessary to obtain a desired coloring or fading, i.e. on the assumption of a maximum coloring/fading, the time necessary for the passage of the amount of charge corresponding to all the potential reaction sites, or, in other words, the time necessary so that intensity I of the current to be reduced to zero--or at least to a value close to zero.
When the size of the glazing increases, the amount of charge that must pass correspondingly increases. However, the intensity of the current cannot increase proportionately because the ohmic drops in large size glazings become a factor limiting the current and consequently the coloring/fading rate with resulting increases in switching times.
While it is desirable to minimize the ohmic drop in a cell operating in transmission--which assumes transparent electroconductive layers--it is not possible to choose materials whose resistance per square is less than, for example, 1 ohm. The best high-performance materials and deposition techniques to date make possible only the production of layers of 2 to 5 ohms of resistance per square. It is possible, however, to obtain some relief from this requirement by using power lead-ins which are not pinpoint but consist of strips or electroconductive wires running along two opposite sides of the cell, masked by a sealing bead forming a framing around the glazing. These strips or wires can be chosen from a very conductive material such as copper, so that all the points of the same strip are equipotential.
In practice, as soon as the distance between the two electroconductive strips exceeds 10 cm, for example, the switching time of a glazing is greater than a minute.
This switching time can be reduced by dividing the glazing into many smaller glazings. Thus, for a glazing of a width W and an electroconductive strip at ordinate y=0 and a second strip at ordinate y=W, the glazing could be subdivided by locating strips at ordinates y=1/3W and y=2/3W by alternating the contact faces. The cell then formed is equivalent to a unit of 3 small identical cells assembled in parallel, each small cell having a resistance three times smaller and an improved switching time. But such an assembly leaves a network of electroconductive strips visible which interferes with the general appearance of the glazing.
What is true of the electrochromic glazings operating in transmission is also true of other electrochromic systems of the display or mirror type which necessarily comprise at least one transparent electroconductive layer whose conductivity is relatively low in comparison, for example, with a thick metal layer.
Further, whatever the mode of operation of the electrochromic system may be--insertion of protons or other cations or the like, for example, reduction/dissolution of a metal salt--, the same problems linked with these ohmic drops are found.